The field of the invention is the production of alkyl esters of saturated aliphatic carboxylic acids and the present invention is particularly concerned with reacting olefins with carbon monoxide and alkanol in the presence of a catalyst consisting of a cobalt compound and a promoter selected from pyridine, non-ortho-substituted alkylpyridine or mixtures thereof at elevated pressures and elevated temperatures.
The state of the art of such alkoxycarbonylation reactions may be ascertained by reference to U.S. Pat. Nos. 3,507,891; 3,906,016; and 4,041,057 and the article "Hydrocarboxymethylation--an Attractive Route from Olefins to Fatty Acid Esters?", by Peter Hofmann et al. as published in I & EC Product Research and Development, Vol. 19, September 1980, pp. 330-334, the disclosures of which are incorporated herein.
It is known that by reacting olefins with carbon monoxide and a compound having a replaceable hydrogen atom such as an alkanol in the presence of a catalyst containing a metal of Group VIII of the Periodic Table of elements and possibly a promoter, fatty acid esters can be produced as disclosed in J. Falbe, Synthesen mit Kohlenmonoxid, Springer publishers, Berling, Heidelberg, New York (1967).
A preferred variation of this reaction, which is termed alkoxycarbonylation, is the conversion in the presence of cobalt catalysts. An especially preferred implementation is found to be the additional use of pyridine, non-ortho-substituted alkylpyridine or mixtures thereof as promoter, as disclosed in U.S. Pat. No. 3,507,891.
A substantive problem in this homogeneously catalyzed reaction is the recovery of the relatively expensive pyridine, non-ortho-substituted alkylpyridine and mixtures thereof from the reaction mixture in such a form as to allow reuse as promoter.
West German Published Application No. 19 63 804 and U.S. Pat. No. 3,507,891 disclose only that the promoter separation can be carried out by distillation.
However, as shown by applicants' own experiments, there is a drop in conversion when a promoter so recovered is reused a number of times. This drop in conversion may be attributed to a deactivation of the catalytic system, mainly by an enrichment in substances acting in an inhibiting manner in the fed back or recycled promoter. However, prior art attempts to remove these inhibitors, manifestly present in low quantities, by distillation at an economically justifiable cost have failed.